Electrolytic capacitors which use metals with a valve effect such as tantalum or aluminum or the like are widely used because a large capacity can be obtained from a small size by making the metal with a valve effect as the electrode for the anode side in the form of a sintered compact or etched foil or the like in order to increase the surface of the dielectric. In particular, solid electrolytic capacitors which use a solid electrolyte for the dielectric material, in addition to being small, having high capacity, and having a low ESR (equivalent series resistance), are easily integrated, and provide characteristics such as being suitable for front surface mounting, and therefore are critical components for reducing the size, increasing performance, and reducing the cost of electronic devices.
When used for small high capacity applications, this type of solid electrolytic capacitor generally has a structure wherein a cathode foil and an anode foil made from a valve effect metal such as aluminum are wound with a separator between to form a capacitor element, this capacitor element is impregnated with a driving electrolytic solution, and the capacitor element is stored in a metal case such as aluminum or a synthetic resin case, and then sealed. Note, aluminum, tantalum, niobium, and titanium or the like are used for the anode material, and the same metals used for the anode material are also used for the cathode material.
Furthermore, manganese dioxide and 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes are known as solid electrolytes which are used in a solid electrolytic capacitors, but in recent years, there has been technology (Japanese Patent Application Laid-open No. H2-15611) which has focused on conductive polymers such as polyethylene dioxythiophene (Hereinafter abbreviated as PEDT) which have a gradual reaction speed and excellent adhesion between the anode electrode and the oxide film layer.
Solid electrolytic capacitors, of the type where a solid electrolyte layer made from a conductive polymer such as PEDT is formed into this type of rolled capacitor element, are manufactured as shown below. First, the surface of an anode foil made from a valve effect metal such as aluminum is roughened by electrochemical etching in a chloride aqueous solution, and after a plurality of etching pits have been formed, a voltage is applied in an aqueous solution such as ammonium borate to form an oxide film layer which becomes the dielectric (anodic forming). Similar to the anode foil, the cathode foil is also made from a valve effect metal such as aluminum, but only etching is performed on this surface.
The anode foil with the oxide film layer formed on the surface in this manner and the cathode foil formed with only etching pits are separated by a separator and rolled to form a capacitor element. Next, a polymeric monomer such as 3,4-ethylenedioxythiophene (Hereinafter referred to as EDT) and an oxidizing agent solution are respectively discharged to the formed capacitor element, or the capacitor element is immersed in a mixture of both solutions, to promote the polymer reaction in the capacitor element and thereby produce a solid electrolyte layer made from a conductive polymer such as PEDT. Next, this capacitor element is stored in a cylindrical external case with a bottom, and the opening of the case is sealed with a sealing rubber to produce a solid electrolytic capacitor.
Incidentally, in recent years, these solid electrolytic capacitors have come to be used for automotive applications. Normally, the drive voltage of an automotive circuit is 12 V, but a solid electrolytic capacitor requires a high voltage proof of 25 V.
Conventionally, these high voltage proof products have been made by using the following method. Namely, products with rated voltage of up to 16 V, the voltage proof can be increased by increasing forming voltage of the foil, but products with rated voltage of above 20 V, electrical shorts frequently occur if the shape is dependent on forming voltage of the foil, so increasing the voltage proof is difficult. Therefore, as a result of investigations into this issue, the present inventors have hypothesized that the point where the oxidizing agent foil is attacked is the limit of the polymer voltage proof (around 20 V on average). Therefore, by using methods which change the formulation ratio of the monomer and the oxidizing agent and improving the separator, they have succeeded in achieving products with rated voltage of 20 V and 25 V.
However, even using the above methods, achieving products with rated voltage of 30 V or 35 V, for which development demand has been increasing in recent years, is difficult.
Furthermore, conventional solid electrolytic capacitors have the following problems in addition to the aforementioned problem.
Namely, in recent years electronic information devices have become digitized and the drive frequency is steadily increasing for the microprocessor units (MPU) which are the heart of these electronic information devices. As a result, the consumption energy is rising and heat induced reliability problems are increasing, so efforts have been made to reduce the drive voltage as a countermeasure.
In order to reduce the drive voltage, DC-DC converters known as voltage control modules are widely used as circuits to provide a highly precise power level to the microprocessor, and a plurality of low ESR capacitors for preventing voltage drop are used as output side capacitors. The aforementioned solid electrolytic capacitors are actually and widely used as capacitors which have low ESR properties.
However, the rise in drive frequency speed for microprocessors has been sharp, and the consumption power has also increased, and as result, there is demand for further increasing the power supplied from the capacitors which prevent a voltage drop. In other words, a large amount of power must be provided in a short period of time, and therefore, the solid electrolytic capacitor is required to have higher capacity, smaller size, and lower voltage, as well as excellent ESR properties.
Note, these problems are not limited to cases where EDT is used as the polymeric monomer, and similarly occur in cases using other thiophene derivatives, pyrroles, and anilines or the like.